It was awful. I was 38 weeks pregnant at the time and had just been told they wanted to do a cs that week because dd was breech. I spent the next 7 days with ds2 in an isolation room at the hospital watching him deteriorate. He had constant diahorrhea (although after not eating for a week he was only passing foam and acid) and I had to carry him to the toilet myself every time. It was only when they changed his medication after a week that he started to turn the corner.
Apparently it was probably the fact that we managed to get him to keep sipping fluids that helped to save him. He would only drink watered down apple juice, but the sugars in that along with the fluids were vital.
He went from a healthy gorgeous happy 4 year old to a weak skinny little thing that was in age 2 clothes and regressed to baby speak and tantrums. He was totally traumatised by the whole thing, suffered terrible nightmares for months afterwards and has become needle phobic thanks to a junior doctor completely messing up his canula insertion - twice!
He is back up to strength physically, although a lot of his hair has recently fallen out (apparently this is common about 3-6 months after severe illness) but we still see the emotional, psychological effects even now.